Mobile smart phones are increasingly being used in situations that call for hands-free communication. As a result, mobile phone users are now using headsets with cables that can be plugged into a headphone jack of the mobile phone. Typical headset designs have a single microphone that is located as near the user's mouth as possible, when the headset is worn, e.g. at the tip of a microphone boom or attached to one of the cables that connect to an earphone of the headset. This positioning however is generally not as good as being very close the user's mouth, often causing a decrease in the signal to noise ratio (SNR) of the speech signal that has been captured or picked up by the microphone. Coupled with the fact that most users often have conversations using their mobile phones in noisy environments, the fidelity of the captured speech signal is often very poor.
One way to improve the performance of sound capture by the headset during a two-way communications or phone call application is to capture the near end user's speech using multiple microphones. This arrangement is also referred to as a microphone array. The processing of microphone array signals improves the SNR of the resulting microphone signal, by spatially filtering the ambient sound field around the near end user. In other words, the array is “pointed” toward the signal of interest, by emphasizing contributions from the better placed microphones while deemphasizing those of the poorly placed microphones. Microphone array processing may also be used in conjunction with a noise cancellation algorithm to more effectively reduce the ambient noise that is captured by the microphones.
Headsets with built-in microphone arrays may also perform better when being used for media playback applications, e.g. while the user is listening to a locally playing MP3 music file, or streaming video over the Internet. In that case, microphone array processing may enhance the noise cancellation algorithm to alleviate the impact of acoustic leakage of ambient noise into an earphone of the headset.
Microphone array processing may be performed within the so-called host device, e.g. the mobile phone handset unit or the digital media player device, by taking advantage of available central processing unit (CPU) data processing power within the host device. To deliver the microphone signals from the headset to the host device, a multi-channel audio codec with a digital microphone interface may be used. The digital microphone interface permits the connection of digital microphones in the headset, to a codec chip within the host device, via a multi-pin cable interface where each pin carries either a single microphone data channel or a reference clock signal (the latter being used for timing purposes to ensure correct sampling of the microphone data signal received by the host device).